At present, a near-bit logging while drilling (LWD) technology has developed rapidly. Compared with a sensor probe of a conventional logging while drilling instrument, a sensor probe of a near-bit logging instrument is closer to a bit, and is capable of relatively immediately acquiring information of a drilled stratum and relatively accurately marking a drilling trajectory so as to reduce the risk of drilling operations and increase the drilling efficiency. In general, the near-bit logging while drilling instrument has three parts: a near-bit measurement sub, a near-bit short-distance transmission apparatus, and a measurement while drilling (MWD) system. The near-bit measurement sub is placed near the bit and is internally provided with an accelerometer, a magnetic sensor and the like. It is configured to measure information of the drilling trajectory. Some systems are further provided with gamma probes and resistivity measurement subs to measure geological information of the drilled stratum in situ. The near-bit short-distance transmission apparatus has an emitter and a receiver, and a screw motor disposed between the emitter and the receiver. A function of transmitting the information of the near-bit measurement sub to the measurement while drilling (MWD) system is realized. Due to its structural limitations, in general the screw motor does not have a wired communication between the emitter and the receiver unless a structure of the screw motor is transformed, and the wired communication is realized by using the through line imbedded in the screw motor. However, this structure is not widely used due to its limitations. Near-bit short-distance transmission is developed towards a wireless transmission manner. Drilling Technology Research Institute of Research Institute of Petroleum Exploration and Development developed a method, which includes a wireless electromagnetic short-distance transmission signal generator carrying a transmitting antenna modulates data acquired by the near-bit measurement sub to generate an electromagnetic signal, and transmits and outputs the electromagnetic signal; a wireless electromagnetic short-distance transmission receiver carrying a receiving antenna receives the transmitted and output electromagnetic signal, demodulates the received electromagnetic signal, and transmits the demodulated data to the MWD system. The measurement while drilling (MWD) system as the third part is mainly composed of a probe, a battery and a mud pulse generator. The near-bit short-distance transmission apparatus transmits the received information, which is measured near the bit, to a ground system by means of the mud pulse generator for real-time monitoring by field engineers.
Implementing near-bit wired short-distance transmission by means of a manner of reconstructing the screw motor (aka mud motor) has its great shortcomings. The screw motor is mainly used in directional wells and in horizontal operations; meanwhile, there are many types of screw motors, and the screw motors used will be different in accordance with different well conditions, hole diameters, stratum environments, and slopes. Further, there are many screw motor manufacturers as well, promoting such a technology of transforming the screw motor and imbedding a through line in a screw motor drilling tool will have a great difficulty, resulting in a small application prospect in the future market.
Short-distance transmission employing the electromagnetic manner basically realizes wireless transmission across the screw motor. Signal transmission is implemented by a transmitting coil and a receiving coil utilizing a Faraday electromagnetic induction principle. However, such an implementation also has shortcomings: an induction signal receives different energies under different grounding resistance environments so that transmitting the induction signal is difficult in a low-resistance stratum. Further, a mechanical processing difficulty is relatively large, the inconvenience is brought to the scene application and the cost is higher. In addition, the wireless electromagnetic short-transmission requires two coils in a housing to serve as the transmitting coil and the receiving coil. The coils have to be completely insulated from the housing, and generally made into annular coils. The processing and installation are very complex. Further, when the contact area between the stratum and mud is small, the transmitted signal cannot be effectively transmitted to the stratum. The transmitting efficiency is low when a transmitting power is increased in order to meet a requirement for an amplitude of receiving the signal. Meanwhile, a signal to noise ratio of the received signal is also relatively poor.